The goal of this project is to investigate the mechanisms by which leptin deficiency regulates susceptibility to intestinal inflammation in mice. In addition to act as a regulator of food intake and energy expenditure, leptin also modulates the immune and inflammatory response. Preliminary data indicate that leptin-deficient (ob/ob) and leptin receptor-deficient (db/db) mice are resistant to colitis induced by chronic administration of DSS or TNBS. The current application will attempt to identify the cell populations responsible for the observed effects. In Specific Aim 1, to evaluate the relative role played by neuronal leptin receptors (Ob-R) in regulation of intestinal inflammation. DSS and TNBS will be administered to mice with a selective deficiency of Ob-R in neurons and their response compared with that of WF mice. To analyze in more detail the role of hypothalamic Ob-R, DSS- and TNBS-induced intestinal inflammation will be studied in mice in which Ob-R have been specifically deleted in the arcuate and ventromedial nuclei of the hypothalamus by injecting a vector expressing cre recombinase into Ob-Rflox/flox mice. Because the effects of leptin on both bone marrow- and non-bone marrow-derived cells could contribute to modulation of colitis in the models of DSS and TNBS administration, in Specific Aim 2 the relative contribution of Ob-R expression in these two cell populations will be investigated using bone marrow chimeras for Ob-R. WT bone marrow will be transplanted into db/db mice, while WT mice will receive db/db bone marrow and colitis development will be studied. Furthermore, the direct effects of leptin on T lymphocytes . Preliminary data indicate that Ob-R expression on donor T lymphocytes regulates induction of colitis in the model of CD4+ CD45RBhigh cells transfer into SCID mice. The transfer model will be employed in the attempt to dissect the relative role played by Ob-R expression in donor T lymphocytes versus cells in the recipient mouse. To investigate the direct role of Ob-R expression in T lymphocytes in the modulation of colitis induced by DSS or TNBS, mice with a selective deletion of Ob-R in T lymphocytes will be employed. Finally, in Specific Aim 3 production of leptin by immune cells at the site of intestinal inflammation will be evaluated in murine models and in biopsies of patients with IBD. The possible participation of immune-derived leptin in the local inflammatory network will also be investigated.